Thermistor reference

Whereas it is no longer an iaterpolation standard of the scale, the thermoelectric principle is one of the most common ways to transduce temperature, although it is challenged ia some disciplines by small iadustrial platinum resistance thermometers (PRTs) and thermistors. Thermocouple junctions can be made very small and ia almost infinite variety, and for base metal thermocouples the component materials are very cheap. Properties of various types of working thermocouple are shown in Table 3 additional properties are given in Reference 5. 

Recommended reference temperature (Curie point) for thermistors (°C) ... 

Zsolnay and Kiel [26] have used flow calorimetry to determine total hydrocarbons in seawater. In this method the seawater (1 litre) was extracted with trichlorotrifluoroethane (10 ml) and the extract was concentrated, first in a vacuum desiccator, then with a stream of nitrogen to 10 pi A 50 pi portion of this solution was injected into a stainless steel column (5 cm x 1.8 mm) packed with silica gel (0.063-0.2 mm) deactivated with 10% of water. Elution was effected, under pressure of helium, with trichlorotrifluoroethane at 5.2 ml per hour and the eluate passed through the calorimeter. In this the solution flowed over a reference thermistor and thence over a detector thermistor. The latter was embedded in porous glass beads on which the solutes were adsorbed with evolution of heat. The difference in temperature between the two thermistors was recorded. The area of the desorption peak was proportional to the amount of solute present. 

Thermocouples The reference junction. As previously mentioned, the resultant reading of a thermocouple depends on the temperature of the two junctions (the measuring and the reference junctions). A reference junction can be made up of an ice bath in which the wires are immersed. A simpler arrangement may be obtained by putting the reference junction (with its two identical copper connections to the voltmeter terminals) in an isothermal block. A thermistor, placed in the same block, measures the absolute temperature of the reference junction, and consequently allows correcting, either by software or hardware compensation, the voltage measured, that will be referenced to 0°C for the subsequent conversion. 

Figure 1. Sensor cell assembly 1, reservoir housing 2, cap 3, support plate 4, M E assembly 5, base plate 6, gasket 1, contact pin 8, thermistor 9, nylon screw 10, Teflon tape 11, gasket 12, gasket 13, counter electrode 14, sensor electrode 15, reference electrode 16, thermistor.

One of the best features of thermal conductivity detectors with helium carrier gas is the ease of quantitative analysis. It has been shown experimentally that relative response factors, where sample weight is used, are independent of (a) type of detector (filament or thermistor), (b) cell and sensor temperature, (c) concentration of sample, (d) helium flowrate, and (e) detector current. In addition, relative response factors change only slightly within a series of homologous compounds. The first systematic study of TCD responses in helium was done by Rosie and Grob and are summarized in reference (6). 

Low-temperature thermistors are usually made from nonstoichiometric iron oxides and have a resistance sensitivity of around 15% per Kelvin at 20 K.17 Thermistors act as ohmic conductors at any fixed temperature. Therefore, one advantage of using a thermistor is that ordinary copper wiring may be used to build the circuit reference junctions and special extension wires are not needed.14 Thermistors are generally quite stable to long-term fluctuations after an initial aging period. 

Fig. 3. Schematic diagram of a CSTR. In the configuration shown, up to three different solutions can be pumped (by the peristaltic pump, PP) into the reactor, R. The detectors shown in the diagram are , light absorption (A/, monochromator PM, photomultiplier), platinum (redox), and iodide (or bromide) selective electrodes. The reference electrode is the Hg/Hg2S04 couple, in place of the usual calomel electrode, to avoid adventitious introduction of chloride into the reactor. In addition to these detectors, a thermocouple, or thermistor, and a pH electrode can be inserted into the reactor from above. The recordings of periodic behavior were taken from studies on the chlorite-iodide reaction...

It is important that the thermistor or RTD used for measuring the cold junction be physically located at the cold junction, as the temperature of the cold junction is often different from that of the room, generally because of heat leakage from the furnace. Special wire, referred to as compensating lead-wire,... 

The most important method of achieving temperature control is to use a sensitive thermometer that generates an electrical signal, such as a resistance thermometer, thermistor, or thermocouple. Comparison of this signal with a reference signal that establishes the set point provides an error signal to a feedback circuit that controls the power to the heater. Thns at r temperature deviation from the desired value detected by the sensor will be corrected antomatically. 

The bolometer is another type of thermal detector that can offer extreme sensitivity for specialized applications, t This is essentially a resistance thermometer, usually with a platinum, nickel, carbon, or germanium element, although a semiconductor thermistor can also be used. Typically, two elements are used in a bridge circuit with one exposed to radiation and the other kept dark as a reference. The germanium bolometer provides exceptional... 

Fig. 4-6. Detector circuits for vapor-phase chromatography, (a) Thermistor detector Di, D=, Victory Eng. Corp. 32A12 thermistors Ri, Ri, 1,0000 wire wound resistors Rz, 1,0000 Helipot Rt, 10,0000 1% carbon film resistor Rs, 5,0000 1 % carbon film resistor Rt, Rj, 2,5000 1 % carbon film resistor Sw, single-pole four-position switch, (b) Hot-wire detector Rz, filament current control, to adjust filament current between 150—300 ma ( 20 ohm 5w) Ri, R4, reference detectors Rs, R, sample detectors Re, zero control 20 Re, 600 1 % carbon film resistor R, 300 1 /, carbon film resistor R, 150 1% carbon film resistor Rse, 7.5Q 1 % carbon film resistor Rs, 7.SCI 1 % carbon film resistor M, 300 ma meter,- S, single-pole six-position sv/itch.

With the catalytic ozone analyzer described, ozone concentration can be measured by the temperature differential between two thermistors placed in the gas stream. One of the thermistors is coated with a catalyst promoting the decomposition of ozone the other is uncoated and is used as reference to the temperature of the gas. The two thermistors are part of a bridge circuit, the output of which is fed directly to a recorder. The instrument is not affected by the presence of water vapor, carbon monoxide, chlorine, nitrogen dioxide, sulfur dioxide, organic peroxides, hydrocarbon vapors, and combustion smokes at their usual concentrations in polluted atmospheres. 

The columns were attached to the end of the plastic tubes by which they are inserted into the calorimeter. Columns could therefore be readily changed with a minimum disturbance of the temperature equilibrium. Inside the plastic tube were the effluent tubing and the leads to the thermistor were fastened to a short piece of gold capillary with heat-conducting, electrically insulating epoxy resin. Veco Type A 395 thermistors (16 kQ at 25 °C, temperature coefficient 3.9%/°C) were used. These are very small, dual-bead isotherm thermistors with 1 % accuracy as such, they were interconvertible, comparatively well matched, and follow the same temperature response curve (within 1 %). An identical thermistor was also mounted in the reference probe. 

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